What was found
CABIN TEMP CONTROL WAS HARDER TO MANAGE IN FLIGHT. NOTICED THE OUTPUT OF THE RT & LT PACKS ARE NOT THE SAME IN STABLE CRUISE. RT PACK IS 6 DEGREES C, LEFT PACK IS 21 DEGREES C. WHEN TRIM AIR IS OFF, RT PACK RETURNS TO 21 DEGREES. ACFT GROUNDED. UNCONTROLLABLE HEAT FROM BELOW 12,000 FT COCKPIT REACHED 54 DEGREES, TEMP CONTROLLER HAD NO IMPACT ON TEMP. TURNING LT PACK OFF ONLY WAY TO GET TEMP TO DECREASE. REPLACED TEMP CONTROL VALVE IAW AMM 21-51-10-000-801. LEAK CHECKED GOOD. ACCOMPLISHED PACK/ZONE TEMP CONTROLLER BITE IAW MM 21-61-00, GROUND VERIFICATION TEST PASSED.